Direct answer
Start with your own chest measurement, find that measurement on the retailer's chart, and then check the garment measurement if one is listed. Knit description, customer photos and the return policy come after those numbers, because they refine a size choice that the measurements have already narrowed down.
Measure yourself before you browse
Online shopping works best when the numbers are settled before you fall for a style. Measure your chest at the fullest point with a soft tape held level, and measure your height and your preferred sweater length if the listing gives a length figure. Write the numbers down, since comparing them with a chart from memory leads to optimistic rounding. Chest is the main driver for sweater size, while length decides whether a style that fits across the chest will also end where you want it to. If you already own a sweater that fits the way you like, measure that garment across the chest while it lies flat and double the figure, since that garment measurement is the most direct comparison you will find for a new purchase.
Read the chart row, not just the label
Size labels hide real differences. One brand's medium may cover a chest of 37 to 39 inches, while another brand's medium may cover 38 to 40 inches, and knit brands often grade more generously than woven shirt brands. Find your chest measurement in the chart and read across the whole row, including sleeve length and body length where they are listed. If your chest falls on a boundary between two rows, note that now rather than forcing a choice, since the knit type and fit description will settle it later. Broad adult chest ranges such as S 34 to 36, M 37 to 39, L 40 to 42 and XL 43 to 45 are useful starting ranges, but they never replace the specific brand chart in front of you, because knit grading varies from brand to brand.
Work through the checks in this order: your measurement, the chart row, the garment measurement, then returns.
What the knit description tells you
The knit description is the next best clue after the numbers. Fine knit usually means a closer, more fitted result that follows the body, while chunky knit means more bulk and a looser visual result even at the same label size. Words like relaxed, oversized, boxy or slim tell you about the intended shape, and they change how a chart row should be read: a relaxed medium may already include the ease you would otherwise seek by sizing up. Fibre content adds a smaller clue, since stretchy blends forgive a tighter chart row and stiff cotton knits forgive less, but do not let fibre override the chart. If the description and the chart disagree, for example a slim fit description on a generously graded chart, trust the chart numbers and treat the description as a warning to check garment measurements.
Photos and reviews, used carefully
Customer photos are useful for shape and drape, and much weaker for size. A photo shows how a sweater hangs, where the hem falls, and whether the knit looks fine or chunky in real light, all of which a flat product photo can hide. Photos help most when the reviewer gives their height, their usual size and the size they bought, because then you can compare their experience with your own measurements. They help least when the review only says the sweater runs large or small with no numbers attached, since one reviewer's large may be another's regular. Read several reviews and look for a pattern, such as repeated comments about long sleeves or a tight neckline, rather than acting on a single strong opinion.
Between sizes and the return safety net
Between sizes is common online, and knit type settles it better than any general rule. For a fitted fine knit, start with the smaller size, because fine knit stretches a little with wear and a smaller starting size keeps the shoulder seam in place. For a relaxed or chunky knit, start with the larger size, because chunky knit holds its shape and a tight chunky sweater feels restrictive across the chest and arms. Length can break the tie: if the smaller size ends shorter than you want, the larger size may be the better trial even in fine knit. Before ordering, check the return policy in full, including whether exchanges are free, how long you have, and whether the sweater must be unworn with tags attached. A simple return policy turns a borderline choice into a low risk trial, and that safety net is worth more than one more hour of comparing photos.
Mistakes that cost an exchange
The costliest online mistake is ordering by label alone, using a size that fits in one brand as if it must fit in another. The second is skipping your own measurement and guessing from a size you wore years ago, since bodies and brands both change. The third is trusting a single review that says a sweater runs small, without checking whether that reviewer measured the same way you do. The fourth is ignoring length, which matters more in knitwear than in many other garments because a sweater that fits across the chest but ends too short cannot be fixed by sizing up for width. Avoid all four by working in order: your measurement, the chart row, the garment measurement, the knit description, photos for shape, and the return policy as a final check.
Sweater sizing tool
Use the Sweater Size Calculator
Enter chest, waist and height measurements to estimate a starting sweater size.
Frequently Asked Questions
What is the first thing to check when buying a sweater online?
Your own chest measurement first, then the retailer's size chart row for that chest measurement, because the label alone does not tell you how that brand grades its knit.
Are garment measurements better than a size chart?
Yes, when they are given. A garment chest measurement tells you how wide the sweater itself is, which you can compare with a sweater you already own and like.
How much should I trust customer photos?
Trust them for shape and drape, not for size. Photos help most when the reviewer lists their height and the size they bought, and least when no measurements are given at all.
What if the sweater is between sizes online?
For a fitted fine knit, start with the smaller size. For a relaxed or chunky knit, start with the larger size, since knit stretches a little but rarely shrinks a whole size smaller.
Why check the return policy before ordering knitwear?
Because knit fit is hard to judge from a photo, and a simple return or exchange turns a borderline size choice into a low risk trial instead of a costly mistake.